The invention relates to a pigtailing method, i.e., the invention provides an optical device comprising a substrate on which are integrated a layered optical waveguide component (3) and optical fiber ends (13). The optical fiber ends are positioned in grooves (10). The method involves providing a substrate (7) comprising grooves (10), notably V-shaped grooves, with a separately made optical waveguide component (3). The component (3) is made on separate flat substrate (1), and released by virtue of a releasable layer (2) present on the flat substrate (1). An advantageous releasable layer is made of a water-soluble salt. The invention also pertains to flexible waveguide sheets such (3) as can be used in the above method or, if provided with waveguide channels (12), as a flexible waveguide component themselves.

1. Field of the Invention
The invention pertains to a method of making an optical device comprising a substrate on which are integrated a layered optical waveguide component comprising a polymeric guiding layer sandwiched between two deflection layers of a lower refractive index than the guiding layer, and optical fibre ends, the optical fibre ends being positioned in grooves.
2. Description of the Related Art
Optical fibre ends attached to the integrated optic device are usually referred to as xe2x80x9cpigtails,xe2x80x9d and the process of providing a waveguide component with such pigtails is referred to as xe2x80x9cpigtailing.xe2x80x9d the invention also pertains to the pigtailed optical waveguide devices so obtainable and to the free-standing, flexible waveguide sheets used in making them.
In general, flexible waveguides are known. E.g., JP 04/232906 discloses a flexible multilayer light guiding sheet suitable for use in signal transmission, e.g., with light splitting such as optical data linking for cars.
JP 05/281428 pertains to a flexible optical interconnection board comprising a flexible optical waveguide mounted onto a base plate that can have various shapes (e.g. curved).
From JP 04/274402 it is known to provide a flexible waveguide by coating a polymeric acid solution onto a substrate, irradiating it so as to produce a precured film, removing said film from the substrate, and curing it so as to form a polyimide waveguide. A polyimide film having a lower index of refraction than the polyimide waveguide film is used to coat at least part of an upper layer and a lower layer of the waveguide film.
It is known to produce polymeric flexible waveguides having a refractive index pattern, see JP 52/138 146 which teaches forming a polymeric film, diffusing a monomer in it, and polymerizing the monomer in selected area. Comparable techniques have been disclosed in JP 78/026813 and JP 54/088144.
From U.S. Pat. No. 4,496,215 it is known to provide an optical interconnection device comprising straight and curved segments, in which the optical interconnection is in the form of fibres (laid in a layered flexible structure).
These references do not specifically address the problem of pigtailing polymeric optical waveguide components. This continues to be one of the principal challenges when making optical waveguide devices. Making a proper connection with a minimum loss of light (coupling loss) between the guiding layer of the waveguide component and the core of the optical fibre is a cumbersome, and generally expensive process step. This especially holds for coupling with single mode optical fibres.
It is known to make use of substrates (usually of silicon) having V-shaped grooves in which fibre ends can be placed. The V-shape of the grooves allows the fibres to be properly aligned vertically (i.e., in respect of the height of the guiding layer) as well as laterally (i.e., in respect of the width of waveguiding channels made in the guiding layer). After the fibres have been placed in the V-grooves they are usually fixed using glue, solder, or the like.
A method as indicated above, employing such a V-grooved substrate, is known from GB 2 000 877. The method disclosed bonding the end-portions of a plurality of optical fibres to V-shaped grooves provided on a transparent substrate by an adhesive; casting a polymer solution onto the fixed optical lead fibres-containing substrate, and evaporating the solvent. Thus, the optical lead fibres are embedded in a polymer layer. Said polymer layer, which serves as the guiding layer of the optical waveguide component, contains, int.al., a photopolymerizable monomer. By selectively activating this monomer, the refractive index of the guiding layer can be selectively decreased. Thus, waveguide channels can be formed in the guiding layer by irradiating the surrounding material. These channels can be made to be aligned with the positions of the embedded ends of the optical fibres. On the guiding layer a low refractive index coating is applied as a top deflection layer. The substrate serves as the lower deflection layer.
The method disclosed in GB 2 000 877 has several drawbacks. In part, these are associated with the use of the substrate as the lower deflection layer. One disadvantage thereof is that the disclosed method cannot be applied to make electro-optically active waveguides. E/O-active, or NLO materials, are known. In such materials non-linear charge polarization occurs under the influence of an external electric field. Non-linear electric polarization may give rise to several optically non-linear phenomena such as frequency doubling and Pockels effect. Obtaining the desired NLO effect in polymeric materials macroscopically requires that first the group present in such a material, mostly hyperpolarizable side groups, be aligned (poled). Such poling is usually effected by exposing the polymeric material to electric (dc) voltage, the so-called poling field, with such heating as will render the polymeric chains sufficiently mobile for orientation. Hence, if a polable e/o material is used in the guiding layer, it is a requirement for poling to occur that the guiding layer can be exposed to an electric field. To this end, two electrodes are needed, one of which is usually applied on top of the layered waveguiding structure, while the other is usually applied at the bottom, i.e., in between the substrate and the lower deflection layer. Providing such a bottom electrode is not envisaged in the method of GB 2 000 877.
Another disadvantage of using the V-grooved substrate as the lower deflection layer is that the refractive index of such a lower deflection layer cannot be tailored to that of the guiding layer and the top deflection layer. Since the difference in refractive indices (the refractive index contrast) of the layers determines the efficiency with which light from an optical fibre can be coupled into the waveguide and vice versa, it is a serious design limitation if the refractive index of any one of the layers cannot be freely chosen. This particularly holds for the deflection layers, since, depending on the nature of the device, the requirements applicable to the guiding layer polymer frequently are more stringent than those applicable to the cladding layers, E.g., in the case of electro-optically active guiding layers, the choice of the guiding layer polymer will be determined more by its e/o coefficient than by other properties such as refractive index. Or, if it is desired to form channels in the guiding layer by means of xe2x80x9cbleachingxe2x80x9d (a photophysical) change of refractive index such as disclosed in EP 358 476), the guiding layer polymer may be chosen for its bleaching capacity more than anything else.
The method of GB 2 000 877 cannot be simply replaced by a method in which a separate lower deflection layer is applicable on the substrate. To begin with, this would undo the very fibre-waveguide alignment as taught in the disclosure. Further, if the lower deflection layer is polymeric, which is highly desirable in respect of tailoring the refractive index and compatibility with the polymeric guiding-layer, the problem applies that the V-grooves will fill up with the deflection layer polymer, leading either to the fibre cores being severely misaligned with the light guiding channels of the polymeric waveguide or unflatness and thickness nonuniformity for the waveguide structure, or both.
Another known method for connecting optical fibres with waveguide devices employing V-grooved Si is known from IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. 13(4), 1990, pages 780-786. The method involves bonding a first substrate, a portion of which carries fibres in V-grooves, with a second substrate carrying an integrated optical circuit, the fibres and the circuit eventually being positioned adjacent to each other. The two substrates face away from one another, and the bonding is achieved by means of solder bumps. This so-called flip-chip solder bump bonding is a self aligning process, the alignment being achieved through the action of surface tension forces during solder reflow. This method has several drawbacks in respect of pigtailing polymeric waveguide components. The required melting and flowing of solder generally requires temperatures that are too high to be used with polymers. A typical temperature being 200xc2x0 C., this will be above the glass transition of many types of electro-optically active polymers, which will lead to these polymers losing their activity. Further, the flux (organic acid that prevents oxidation of the solder) that as a rule is dispensed on the solder during reflow, is likely to attack the polymer. Further the method has the drawback of the final product comprising no less than three different substrates: the two substrates mentioned, i.e., one carrying the integrated optical circuit and the other carrying the fibres in V-grooves, together need be supported by a third substrate, a so-called motherboard.
The invention now seeks to provide a pigtailing method that can be used with passive as well as thermo-optically and electro-optically active waveguides, allows refractive index tailoring, and can be used with polymers. In providing such a method, it is an object of the invention to avoid the complicated use of three different substrates and to wind up with a device comprising a single substrate. To this end, the invention consists in that in a method as described in the opening paragraph, a substrate comprising grooves, notably V-shaped grooves, is provided with a separately made polymeric structure comprising one or more layers, hereinafter referred to a free-standing sheet.
The free-standing sheet can either be a single layer, or it can be a layered structure comprising the guiding layer and at least one deflection layer, said layered structure being applied with the deflection layer facing the substrate. Preferably, the free-standing sheet is a complete layered waveguide structure comprising the guiding layer and two deflection layers enclosing the guiding layer.
In the case of the free-standing sheet being a single layer, it serves to planarize the V-grooved substrate. After this layer has been applied, other layers can be applied onto it, by means of spincoating, evaporating etc. The spincoating process, which is generally recognized to be the most convenient method for applying thin layers, such as in the case of polymeric optical waveguides, as a rule involves rapidly rotating the entire substrate to be coated. Hence, the further application of layers will necessarily involve covering the entire grooved substrate. The planarizing layer (which may serve as the lower deflection layer or may be an additional layer) is therefore applied over the entire grooved surface. After the other layers of the layered waveguide structure have been applied, a portion thereof will have to be removed (including the corresponding portion of the planarizing layer) so as to uncover the grooves. Such removal will generally have to be done by means of etching (e.g. O2 reactive ion etching). The use of a single planarizing layer has as its main advantage that a polymeric layered waveguide structure can be applied onto a grooved substrate without the grooves filling up with polymer. Since the further spincoating steps are conventional, and allow the formation of a well-defined layered waveguide structure, proper alignment of fibres and waveguide may be attained. However, employing a free-standing sheet in the form of a complete layered waveguide structure has important additional advantages. This embodiment is less complicated, the waste of material is reduced to a minimum, and a smoother waveguide facet can be made. In all its embodiments, the invention allows obtaining an accurate connection of the waveguide with the optical fibres by adapting the layer thickness of the layered waveguide structure (and, if used, the planarizing layer) to the dimensions of the optical fibres (notably: core diameter). Thus, the cores of the optical fibres (usually cylindrical or elliptical) can be positioned face to face with the guiding layer of the layered waveguide structure.
The method of the invention involves separately providing a grooved substrate already containing fibre ends or not, and making a free-standing sheet.
The manner in which the grooved substrate is provided is not in itself part of the present invention. Instead, such substrates are commercially available or can be made by methods known to the man skilled in the art.
In principle, the substrate can be made of any material suitable for supporting a polymeric waveguide structure. Examples are thermoset polymers, in which the desired grooves can be made, e.g., by shaping techniques such as injection moulding, glass, or other inorganic materials such as silicon or GaAs and the like. In the latter materials, grooves can be made by etching techniques known from the semiconductor industry. In this respect silicon is particularly preferred. The grooves preferably are V-shaped, thus providing two facets in respect of which optical fibres can be very well kept in place, thus allowing positioning in respect of waveguide facets most accurately. The depth of the grooves logically depends on the diameter of the optical fibres employed.
If use is made of single mode fibres, which have a core diameter of about 9 xcexcm, the substrate preferably is a monocrystalline material exhibiting an anisotropic etching characteristic, since high accuracy in the dimensions of the V-grooves which will contain the fibres having a diameter of the order of 125 micron must be obtained. In this respect silicon, gallium arsenide and indium phosphide are examples of suitable substrate materials.
If the free-standing sheet is not a mere planarizing layer, the grooved substrate can be provided with fibre ends before the free-standing sheet is applied. It is preferred to provide the fibre ends afterwards. In the case of an electro-optically active waveguide being formed, a bottom electrode in the form of a thin layer of metal is applied first, at least at the portion of the substrate that is to be covered with the free-standing sheet. This bottom metallization, preferably gold, is usually applied by evaporation or sputtering. Particularly in this embodiment, it is preferred if the substrate comprises two portions: a portion containing grooves (in which eventually the fibres will be laid) and a flat portion, on which first the metal layer and next the free-standing sheet is applied.
The free-standing sheet is prepared separately. In view of the required accuracy and thickness uniformity of the layers, or single layer, it is generally necessary to first make the free-standing sheet on a substrate. Such a substrate will be planar (flat), so that a layer, or layered structure, having clearly defined layer thicknesses can be made. After the layer or the layered structure has been lifted off the original substrate, it can be placed on the grooved substrate. The original substrate can be made of the above-mentioned substrate materials, silicon being preferred.
The polymeric materials from which the free-standing sheet, and eventually the layered waveguide are built up are not crucial to the present invention.
The deflection layers, as well as the guiding layer in the case of a passive optical component, can be made of any suitable polymer which is transparent to the wavelength employed and displays minimal optical losses for the desired wavelength. The most important wavelength ranges in actual practice are approx. 670 nm, from about 800 to about 1000 nm, approx. 1300 nm, and approx. 1500 nm. These last two wavelength ranges are especially suited for use in telecommunications. Of course, it is imperative that the guiding layer have a higher index of refraction than the deflection layers. The man skilled in the art will be aware of optical polymers that can be tuned to have the desired refractive index. Examples of suitable deflection (cladding) layer materials are acrylate-styrene copolymers, such as copolymers of hydroxyethyl methacrylate and styrene, and the same in conjunction with maleic anhydride. It may be advantageous to apply these polymers in the form of a prepolymer having functional end groups, and to include a polyfunctional cross-linking agent for these end groups (e.g. hydroxyl end groups to be cross-linked with Desmodur-N triisocyanate). The suitable polymeric materials for passive waveguides are known to the skilled person. Further, it is of importance for the core and deflection layers to be easy to treat and so processable as to give the flattest and purest possible contact surfaces between the core and deflection layers as well as sufficient adhesion between the core and deflection layers. The skilled person will know which materials to select to this end. Preferably, use is made of materials in which waveguide channels can be made by means of irradiation, as described below.
As indicated before, in electro-optically active waveguides use is made of polymers having a non-linear optical activity macroscopically effected therein by means of alignment. Suitable NLO-active materials (NLO polymers) have been described in EP 358 476 and in, e.g., EP 350 112, EP 350 113, EP 359 648, U.S. Pat. No. 4,867,540, U.S. Pat. No. 4,795,664, and WO 91/03001. For this matter, these materials are also suitable for use in passive waveguides.
Making the free-standing sheet in the form of a layered waveguide structure will generally involve the following steps:
Providing a flat substrate with a layer that is releasable upon proper treatment, e.g., a gold layer, or a layer of water-soluble material such as polyvinyl alcohol or water-soluble salt;
Applying the first deflection layer polymer on the releasable layer;
Applying the guiding layer polymer on the first deflection layer;
Applying the second deflection layer polymer on the guiding layer;
Releasing the layered structure from the flat substrate by suitably treating the releasable layer; in the case of a gold layer such treatment will involve subjection to warm KOH (appr. 50xc2x0 C.)
In the case of the free-standing sheet comprising less than a full layered waveguide structure, the appropriate steps can be omitted. If the free-standing sheet is a separate planarizing that does not serve as the first deflection layer, the layer applied on the releasable layer can basically be any polymeric layer.
According to a particularly advantageous embodiment of the invention, the releasable layer comprises a water-soluble material such as a salt such as NaCl or polyvinyl alcohol. This means that agressive treatment such as subjection to warm KOH can be avoided, and the waveguide sheet can be released from the original substrate by dipping the entire structure in water. It is then ready to be placed on the grooved substrate.
In some cases, particularly when the waveguide structure is so flexible as to give handling problems, an alternative to first releasing the waveguide sheet and then placing it on the grooved substrate is to perform these acts simultaneously. This can be done by placing the grooved substrate on top of the releasable waveguide structure still on the original substrate, and then activating the releasable layer. The grooved side of the substrate has to face the deflection layer. The original top deflection layer thus becomes the bottom deflection layer on the grooved substrate. Although this has been found to work without further measures, it may be advantageous to enhance the adhesion of the waveguide structure to the grooved substrate placed on top of it by first providing the grooved substrate with a thin adhesive layer. This can most conveniently be layer of about 0.5 xcexcm thickness of the same polymeric material as the deflection layer. Such a thin layer also covers the bottom of the grooves, but does not fill them up. Hence, the above-identified drawback that occurs when a full cladding layer (having a thickness of the order of 3-15 xcexcm) is applied on the grooved substrate does not occur. Preferably, the adhesive layer is a still curable prepolymer, which is cured (by applying heat) after the structure has been released from the original substrate. In respect of thickness, it may be mentioned that the three-layered waveguide sheet will generally have a thickness of from about 5 to about 50 xcexcm.
An advantage of first making a clearly defined structure of a polymeric guiding layer sandwiched in between two polymeric cladding layers is that the placing of the structure on the grooved substrate does not require high accuracy. For, this waveguide sheet can be applied on the groove substrate with relatively low precision, and may cover an area extending beyond that on which the waveguide component eventually is desired. After the waveguide sheet has been so applied, and the channels are defined and a smooth facet is etched at the side at which the optical fibres will be positioned. All protruding parts can be removed using any appropriate etching technique, e.g., those known from the production of integrated circuits (ICs). Applicable in this case are wet-chemical etching techniques, e.g., with use being made of organic solvents or strong bases. However, preference is given to dry etching techniques, such as sputter etching, reactive ion etching, or reactive plasma etching. Such techniques are known to the skilled person and require no further elucidation here. Alternatively, there may be etching using a laser, mechanical etching such as grinding, cutting, drilling, or etching through bombardment with sanding particles such as alumina, silica, and, more particularly, pumice. The skilled person is expected to be able to select an appropriate etchant without undue experimentation. Preferably, use is made of reactive ion etching (RIE) or a combination of RIE and reactive plasma etching, while so-called laser ablation is another very suitable technique. To make the facet, and to remove the protruding portions of the layered polymeric structure, a mask is applied to cover those parts which should remain free from attack by the etchant. These masks, the chief prerequisite of which is that they be resistant to the action of the etchant, are known, int. al., from IC technology. Such a mask be preformed and made up of, e.g., metal or synthetic material; alternatively, it can be made by applying a photosensitive resin (photoresist) and subsequently exposing and developing said resin in accordance with the desired pattern.
If an air gap between the optical fibres and the waveguide cannot be avoided simply by accurate positioning of the fibres and the waveguide, a filler, preferably low viscous, may be used to fill the gap. In this way, providing the optical properties of the filler do not differ markedly from those of the polymer, an inconvenient refractive index contrast in relation to an air gap may be avoided. Preferably the filler selected has the same, or virtually the same, refractive index as the core layer of the polymeric waveguide. Thus the polymer itself can very well be used as a filler, either in the form of a solution or (in the case of a thermoset) in the uncured state. Alternatively, use may be made of oligomers analogous to the polymer, a glue, or some other adhesive. Suitable adhesives are for instance the glues of the well-known epoxy or cyanoacrylate types.
In the case of an electro-optically active waveguide being prepared, the preferred method of the invention involves applying a first electrode in the form of a metal layer (preferably gold) on the salt layer, applying the first deflection layer, the guiding layer, and the second deflection layer, and then applying a second electrode in the form of a metal layer on the second deflection layer. Now the waveguide structure can be poled before it is released, by applying a suitable voltage to the two electrodes. Thereafter, the poled layered waveguide structure is released from the flat substrate, and applied on a guest substrate.
Usually, it will be necessary to define waveguide channels (i.e., lateral wave confinements) in the guiding layer. Methods of achieving this are known. For instance, such patterns may be provided by removing portions of the layered waveguide structure, e.g., by means of wet-chemical or dry etching techniques, and filling the formed voids with a material having a lower index of refraction (thus forming a wave confinement in the form of a channel made up of material enclosed on all sides by deflection layer material).
Alternatively, it is possible to use photosensitive material, which can be developed after irradiation; for instance, a negative photoresist, that is to say, material which is resistant to a particular solvent (developer) after being irradiated. The developer in that case may be used to remove non-irradiated material. However, it is preferred to employ a positive photoresist and have the developer remove the portion that has been irradiated.
The preferred technique, however, involves making use of a core material in which a waveguide pattern can be provided without any material being removed by etching. For instance, there is core material which is chemically converted into a material with a different index of refraction under the influences of heat, light or UV irradiation. If this concerns an increase in the index of refraction the treated material will be used as core material. This may take the form of carrying out the treatment using a mask, with the holes in the mask being identical with the desired waveguide pattern. If, on the other hand, a reduction of the index of refraction is involved, the treated material will be suited for use as deflection material. The treatment in question in that case may be carried out using a mask of which the closed portions are identical with the desired waveguide pattern, See, e.g., the aforementioned GB 2 000 877.
In the process according to the present invention it is preferred to employ a flat waveguide of which the core layer comprises a polymer bleachable under the influence of irradiation. This is a particular type of light- or UV-sensitive core layer material. Probably because of a chemical rearrangement reaction, irradiation, preferably generally using blue light, lowers the index of refraction of such a material without affecting the remaining physical and mechanical properties. Preferably, the flat waveguide is provided with a mask covering the desired pattern of channels, so that the surrounding material can have its index of refraction lowered (xe2x80x9cbe bleachedxe2x80x9d) by means of irradiation. Thus, as desired, waveguide channels are formed which are enclosed on all sides by material having a lower index of refraction (the bottom and top deflection layers and the surrounding bleached core layer material). Such bleachable polymers have been described, int.al., in EP 358 476.
In the method of the present invention, it is particularly advantageous to use the same mask material first for etching the facet and removing protruding portions of the layered waveguide structure, and next for irradiating to form the desired waveguide channels. Thus, after transplanting the waveguide sheet from the original substrate to the grooved guest substrate, e.g. a gold layer can be applied on the entire waveguide structure. Then, using a conventional gold etchant such as KI/I2, the gold is removed by etching exactly along the boundaries of the grooved substrate (thus leaving open the protruding portions of the transplanted waveguide) and at the position of the desired waveguide facet. Defining which gold is to be removed by etching and which gold is to be kept can be conveniently done by means of a photoresist mask. All the redundant, protruding, layered waveguide material devoid of the gold layer is then removed by etching. thus leaving a grooved substrate a portion of which is provided with a layered waveguide structure having a smooth facet, the layered waveguide structure still being covered with a layer of gold. This same layer of gold can then be partially removed by etching, again employing a photomask and again using KI/I2, so as to define a pattern of areas that should be bleached. Since only a portion of the substrate is covered with the waveguide structure, the grooves lie free, and are visible, on the other portion (adjacent to the etched facet). The visibility of the grooves enables accurate positioning of the desired waveguide channels, i.e., lateral alignment with the grooves (hence with the optical fibres). Accurate positioning can also be obtained by aligning corresponding alignment marks on both the grooved substrate and the mask for waveguide channel definition. It is also possible to have facets at either end of the waveguide structure, and to have a free portion of the grooved substrate adjacent to each facet.
Alternatively, the waveguide sheets can be provided with lateral wave confinements (channels) prior to their transplantation from the original, flat substrate to the grooved guest substrate. Of course, this will not be as advantageous in respect of alignment with the optical fibers, but the resulting free-standing waveguide sheets may very well be applied in somewhat different manner, viz. as a flexible optical waveguide component. I.e., the free-standing sheet is not transplanted to a guest substrate, but used as is, thus making favourable use of the free-standing waveguide sheet""s property of flexibility.
Thus, the invention also pertains to an optical waveguide component having a layered structure comprising a polymeric guiding layer sandwiched between two deflection layers of a lower refractive index than the guiding layer, with the guiding layer having been provided with lateral wave confinements, wherein the layered structure is pliable and not fixed on a rigid substrate. Such flexible waveguide components, which are basically the same as the waveguide sheets used in the above-described pigtailing method, and manufactured in the same manner but provided with waveguide channels, can be advantageously applied as a flexible optical interconnect (the optical counterpart of the well-known flexible printed circuit board). E.g., the flexible waveguide components allow making an interconnection between two or more optical devices or boards, while passing over and under other optical or electronic structures that may be present in between such devices or boards. As compared to the known fibre-ribbons, which find similar usage, the present flexible polymeric optical waveguide components have the advantage of allowing a much higher interconnection density and easier alignment of the waveguide array. The loss induced by bending the waveguides over radii of curvature of a few mm (say, about 2-10 mm) is negligible. It is also envisaged to make a coiled up flexible waveguide. As background literature on such flexible optical waveguide components are mentioned the publication of Bruce L. Booth in Polymers for electronic and photonic applications, ed. C. P. Wong, Academic Press, San Diego 1993, pages 549-599, and in Journal of Lightwave Technology, vol. 7, No. 10, October 1989, pages 1445-1453.
In a particularly advantageous embodiment, it is possible to look through the flexible polymeric layered waveguide structure can be viewed through with light (visible or infrared) microscopy. In this embodiment, the waveguide channels can be precisely aligned with patterns on a substrate on which fibre ends, detectors, or light sources are accurately mounted in respect of these patterns. Most of the polymers used for guiding light are transparent within a range of from 530 nm (yellow) to 1700 nm (infrared).
The end portions of the flexible waveguide component will usually be provided with rigid covers (e.g., by covering both deflection layers with a thin glass plate at both ends of the component). A typical thickness for such a glass plate is 400 xcexcm, a typical length is 1-5 mm.
Preferably, the flexible waveguide component comprises, on both surfaces, a protective coating layer, i.e., the layered waveguide structure is sandwiched between protective coating layers. The above-mentioned rigid covers can then be applied over this protective coating. The protective coating preferably is an additional, relatively thick polymer layer (typically from about 50 to about 500 xcexcm, preferably about 200-300 xcexcm) fixed on the waveguide structure by means of an adhesive layer. The protective coating can be applied in the form of conventional, transparent, adhesive tape. It is particularly preferred for such a flexible optical waveguide component to comprise an interconnection section (i.e. the end of ends of the component at which a coupling with optical fibres has to be made) which is devoid of one of the layers of protecting coating. The length of such an interconnection section will generally be as above, i.e., about 1-5 mm. At the end of the interconnection section a smooth facet is fabricated to allow the coupling in and coupling out of light. This facet can be fabricated by the previously described etching techniques, by polishing techniques, or by a knife-cut (microtoming). An interconnection section of this type has the considerable advantage of making it possible to accurately align the flexible waveguide to optical fibers. This alignment is still further improved if the interconnection section is provided with a rigid cover, such as a thin glass plate, on the side facing away from the side devoid of the protective coating. Thus, the interconnection section permits easy handling and basically can be moved in lateral, vertical, and longitudinal direction in respect of the optical waveguide components (such as optical fibres, laser diodes, detectors, waveguides) to which coupling should occur.

Method of making a polymeric optical waveguide device provided with fibre ends, and free-standing, flexible waveguide sheets used therein


The invention relates to an imaging member comprising an imaging layer and a base wherein said base comprises a closed cell foam core sheet and adhered thereto an upper and lower flange sheet, and wherein said imaging member has a stiffness of between 50 and 250 millinewtons.

This invention relates to imaging media. In a preferred form, it relates to supports for photographic, ink jet, thermal, and electrophotographic media.
In order for a print imaging support to be widely accepted by the consumer for imaging applications, it has to meet requirements for preferred basis weight, caliper, stiffness, smoothness, gloss, whiteness, and opacity. Supports with properties outside the typical range for xe2x80x98imaging mediaxe2x80x99 suffer low consumer acceptance.
In addition to these fundamental requirements, imaging supports are also subject to other specific requirements depending upon the mode of image formation onto the support. For example, in the formation of photographic paper, it is important that the photographic paper be resistant to penetration by liquid processing chemicals failing which there is present a stain on the print border accompanied by a severe loss in image quality. In the formation of xe2x80x98photo-qualityxe2x80x99 ink jet paper, it is important that the paper is readily wetted by ink and that it exhibits the ability to absorb high concentrations of ink and dry quickly. If the ink is not absorbed quickly, the elements block (stick) together when stacked against subsequent prints and exhibit smudging and uneven print density. For thermal media, it is important that the support contain an insulative layer in order to maximize the transfer of dye from the donor which results in a higher color saturation.
It is important, therefore, for an imaging media to simultaneously satisfy several requirements. One commonly used technique in the art for simultaneously satisfying multiple requirements is through the use of composite structures comprising multiple layers wherein each of the layers, either individually or synergistically, serves distinct functions. For example, it is known that a conventional photographic paper comprises a cellulose paper base that has applied thereto a layer of polyolefin resin, typically polyethylene, on each side, which serves to provide waterproofing to the paper and also provides a smooth surface on which the photosensitive layers are formed. In another imaging material as in U.S. Pat. No. 5,866,282, biaxially oriented polyolefin sheets are extrusion laminated to cellulose paper to create a support for silver halide imaging layers. The biaxially oriented sheets described therein have a microvoided layer in combination with coextruded layers that contain white pigments such as TiO2 above and below the microvoided layer. The composite imaging support structure described has been found to be more durable, sharper, and brighter than prior art photographic paper imaging supports that use cast melt extruded polyethylene layers coated on cellulose paper. In U.S. Pat. No. 5,851,651, porous coatings comprising inorganic pigments and anionic, organic binders are blade coated to cellulose paper to create xe2x80x98photo-qualityxe2x80x99 ink jet paper.
In all of the above imaging supports, multiple operations are required to manufacture and assemble all of the individual layers. For example, photographic paper typically requires a paper-making operation followed by a polyethylene extrusion coating operation, or as disclosed in U.S. Pat. No. 5,866,282, a paper-making operation is followed by a lamination operation for which the laminates are made in yet another extrusion casting operation. There is a need for imaging supports that can be manufactured in a single in-line manufacturing process while still meeting the stringent features and quality requirements of imaging bases.
It is also well known in the art that traditional imaging bases consist of raw paper base. For example, in typical photographic paper as currently made, approximately 75% of the weight of the photographic paper comprises the raw paper base. Although raw paper base is typically a high modulus, low cost material, there exist significant environmental issues with the paper manufacturing process. There is a need for alternate raw materials and manufacturing processes that are more environmentally friendly. Additionally to minimize environmental impact, it is important to reduce the raw paper base content, where possible, without sacrificing the imaging base features that are valued by the customer, i.e., strength, stiffness, surface properties, etc. of the imaging support.
An important corollary of the above is the ability to recycle photographic paper. Current photographic papers cannot be recycled because they are composites of polyethylene and raw paper base and, as such, cannot be recycled using polymer recovery processes or paper recovery processes. A photographic paper that comprises significantly higher contents of polymer lends itself to recycling using polymer recovery processes.
Existing composite color paper structures are typically subject to curl through the manufacturing, finishing, and processing operations. This curl is primarily due to internal stresses that are built into the various layers of the composite structure during manufacturing and drying operations, as well as during storage operations (core-set curl). Additionally, since the different layers of the composite structure exhibit different susceptibility to humidity, the curl of the imaging base changes as a function of the humidity of its immediate environment. There is a need for an imaging support that minimizes curl sensitivity as a function of humidity, or ideally, does not exhibit curl sensitivity.
The stringent and varied requirements of imaging media, therefore, demand a constant evolution of material and processing technology. One such technology known in the art as xe2x80x98polymer foamsxe2x80x99 has previously found significant application in food and drink containers, packaging, furniture, appliances, etc. Polymer foams have also been referred to as cellular polymers, foamed plastic, or expanded plastic. Polymer foams are multiple phase systems comprising a solid polymer matrix that is continuous and a gas phase. For example, U.S. Pat. 4,832,775 discloses a composite foam/film structure which comprises a polystyrene foam substrate, oriented polypropylene film applied to at least one major surface of the polystyrene foam substrate, and an acrylic adhesive component securing the polypropylene film to said major surface of the polystyrene foam substrate. The foregoing composite foam/film structure can be shaped by conventional processes as thermoforming to provide numerous types of useful articles including cups, bowls, and plates, as well as cartons and containers that exhibit excellent levels of puncture, flex-crack, grease and abrasion resistance, moisture barrier properties, and resiliency.
Foams have also found limited application in imaging media. For example, JP 2839905 B2 discloses a 3-layer structure comprising a foamed polyolefin layer on the image-receiving side, raw paper base, and a polyethylene resin coat on the backside. The foamed resin layer was created by extruding a mixture of 20 weight % titanium dioxide master batch in low density polyethylene, 78 weight % polypropylene, and 2 weight % of Daiblow PE-M20 (AL)NK blowing agent through a T-die. This foamed sheet was then laminated to the paper base using a hot melt adhesive. The disclosure JP 09127648 A highlights a variation of the JP 2839905 B2 structure, in which the resin on the backside of the paper base is foamed, while the image receiving side resin layer is unfoamed. Another variation is a 4-layer structure highlighted in JP 09106038 A. In this, the image receiving resin layer comprises of 2 layers, an unfoamed resin layer which is in contact with the emulsion, and a foamed resin layer which is adhered to the paper base. There are several problems with this, however. Structures described in the foregoing patents need to use foamed layers as thin as 10 xcexcm to 45 xcexcm, since the foamed resin layers are being used to replace existing resin coated layers to the paper base. The thickness restriction is further needed to maintain the structural integrity of the photographic paper base since the raw paper base is providing the stiffness. It is known by those versed in the art of foaming that it is very difficult to make thin uniform foamed films with substantial reduction in density especially in the thickness range noted above.
There is a need for a composite material that can be manufactured in a single in-line operation and that meets all the requirements of an imaging base.
There is also a need for an imaging base that reduces the amount of raw paper base that is used.
There is also a need for an imaging base that can be effectively recycled.
There is also a need for an imaging base that resists the tendency to curl as a function of ambient humidity.
It is an object of the invention to provide a composite imaging material that overcomes the disadvantages of prior imaging base.
It is a further object of this invention to provide a composite imaging material that resists humidity curl.
It is another object to provide an imaging member that can be manufactured in-line in a single operation.
It is another further object to provide an imaging member that can be recycled.
These and other objects of the invention are accomplished by an imaging member comprising an imaging layer and a base wherein said base comprises a closed cell foam core sheet and adhered thereto an upper and lower flange sheet, and wherein said imaging member has a stiffness of between 50 and 250 millinewtons.
This invention provides a superior imaging support. Specifically, it provides an imaging support of high stiffness, excellent smoothness, high opacity, and excellent humidity curl resistance. It also provides an imaging support that can be manufactured using a single in-line operation. It also provides an imaging support that can be effectively recycled.
This invention has numerous advantages. The invention produces an element that has much less tendency to curl when exposed to extremes in humidity. The element can be manufactured in a single in-line operation. This significantly lowers element manufacturing costs and would eliminate disadvantages in the manufacturing of the current generation of imaging supports including very tight moisture specifications in the raw base and specifications to minimize pits during resin coating. The element can also be recycled to recover and reuse polyolefin instead of being discarded into landfills. It is an objective of this invention to use foam at the core of the imaging base, with high modulus flange layers that provide the needed stiffness surrounding the foam core on either side. Using this approach, many new features of the imaging base may be exploited and restrictions in manufacturing eliminated. These and other advantages will be apparent from the detailed description below.
The imaging member of the invention comprises a polymer foam core that has adhered thereto an upper and a lower flange sheet. The polymer foam core comprises a homopolymer such as a polyolefin, polystyrene, polyvinylchloride or other typical thermoplastic polymers; their copolymers or their blends thereof; or other polymeric systems like polyurethanes, polyisocyanurates that has been expanded through the use of a blowing agent to consist of two phases, a solid polymer matrix, and a gaseous phase. Other solid phases may be present in the foams in the form of fillers that are of organic (polymeric, fibrous) or inorganic (glass, ceramic, metal) origin. The fillers may be used for physical, optical (lightness, whiteness, and opacity), chemical, or processing property enhancements of the foam.
The foaming of these polymers may be carried out through several mechanical, chemical, or physical means. Mechanical methods include whipping a gas into a polymer melt, solution, or suspension, which then hardens either by catalytic action or heat or both, thus entrapping the gas bubbles in the matrix. Chemical methods include such techniques as the thermal decomposition of chemical blowing agents generating gases such as nitrogen or carbon dioxide by the application of heat or through exothermic heat of reaction during polymerization. Physical methods include such techniques as the expansion of a gas dissolved in a polymer mass upon reduction of system pressure; the volatilization of low-boiling liquids such as fluorocarbons or methylene chloride, or the incorporation of hollow microspheres in a polymer matrix. The choice of foaming technique is dictated by desired foam density reduction, desired properties, and manufacturing process.
In a preferred embodiment of this invention polyolefins such as polyethylene and polypropylene, their blends and their copolymers are used as the matrix polymer in the foam core along with a chemical blowing agent such as sodium bicarbonate and its mixture with citric acid, organic acid salts, azodicarbonamide, azobisformamide, azobisisobutyrolnitrile, diazoaminobenzene, 4,4xe2x80x2-oxybis(benzene sulfonyl hydrazide) (OBSH), N,Nxe2x80x2-dinitrosopentamethyltetramine (DNPA), sodium borohydride, and other blowing agent agents well known in the art. The preferred chemical blowing agents would be sodium bicarbonate/citric acid mixtures, azodicarbonamide; though others can also be used. If necessary, these foaming agents may be used together with an auxiliary foaming agent, nucleating agent, and a cross-linking agent.
The flange sheets of this invention are chosen to satisfy specific requirements of flexural modulus, caliper, surface roughness, and optical properties such as colorimetry and opacity. The flange members may be formed integral with the foam core by manufacturing the foam core with a flange skin sheet or the flange may be laminated to the foam core material. The integral extrusion of flange members with the core is preferred for cost. The lamination technique allows a wider range of properties and materials to be used for the skin materials. Imaging elements are constrained to a range in stiffness and caliper. At stiffness below a certain minimum stiffness, there is a problem with the element in print stackability and print conveyance during transport through photofinishing equipment, particularly high speed photoprocessors. It is believed that there is a minimum cross direction stiffness of 60 mN required for effective transport through photofinishing equipment. At stiffness above a certain maximum, there is a problem with the element in cutting, punching, slitting, and chopping during transport through photofinishing equipment. It is believed that there is a maximum machine direction stiffness of 300 mN for effective transport through photofinishing equipment. It is also important for the same transport reasons through photofinishing equipment that the caliper of the imaging element be constrained between 75 xcexcm and 350 xcexcm.
Imaging elements are typically constrained by consumer performance and present processing machine restrictions to a stiffness range of between approximately 50 mN and 250 mN and a caliper range of between approximately 100 xcexcm and 400 xcexcm. In the design of the element of the invention, there exists a relationship between stiffness of the imaging element and the caliper and modulus of the foam core and modulus of the flange sheets, i.e., for a given core thickness, the stiffness of the element can be altered by changing the caliper of the flange elements and/or changing the modulus of the flange elements and/or changing the modulus of the foam core.
If the target overall stiffness and caliper of the imaging element are specified then for a given core thickness and core material, the target caliper and modulus of the flange elements are implicitly constrained. Conversely, given a target stiffness and caliper of the imaging element for a given caliper and modulus of the flange sheets, the core thickness and core modulus are implicitly constrained.
Preferred ranges of foam core caliper and modulus and flange caliper and modulus follow: the preferred caliper of the foam core of the invention ranges between 200 xcexcm and 350 xcexcm, the caliper of the flange sheets of the invention ranges between 10 xcexcm and 175 xcexcm, the modulus of the foam core of the invention ranges between 30 MPa and 1000 MPa, and the modulus of the flange sheets of the invention ranges from 700 MPa to 10500 MPa. In each case, the above range is preferred because of (a) consumer preference, (b) manufacturability, and (c) materials selection. It is noted that the final choice of flange and core materials, modulus, and caliper will be a subject of the target overall element stiffness and caliper.
The selection of core material, the extent of density reduction (foaming), and the use of any additives/treatments for, e.g., cross-linking the foam, determine the foam core modulus. The selection of flange materials and treatments (for example, the addition of strength agents, etc. for paper base or the use of filler materials for polymeric flange materials) determines the flange modulus.
For example, at the low end of target stiffness (50 mN) and caliper (100 xcexcm), given a typical polyolefin foam of caliper 50 xcexcm and modulus 137.9 MPa, the flange sheet caliper is then constrained to 25 xcexcm on each side of the core, and the flange modulus required is 10343 MPa, properties that can be met using a high modulus paper base. Also, for example, at the high end of target stiffness (250 mN) and caliper (400 xcexcm), given a typical polyolefin foam of caliper 300 xcexcm and modulus 137.9 MPa, the flange sheet caliper is constrained to 50 xcexcm on each side and the flange modulus required is 1034 MPa, properties that can be met using a polyolefin flange sheet.
In a preferred lamination embodiment of this invention, the flange sheets used comprise paper. The paper of this invention can be made on a standard continuous fourdrinier wire machine or on other modem paper formers. Any pulps known in the art to provide paper may be used in this invention. Bleached hardwood chemical kraft pulp is preferred, as it provides brightness, a good starting surface, and good formation while maintaining strength. Paper flange sheets useful to this invention are of caliper between about 25 xcexcm and about 100 xcexcm, preferably between about 30 xcexcm and about 70 xcexcm because then the overall element thickness is in the range preferred by customers for imaging element and processes in existing equipment. They must be xe2x80x9csmoothxe2x80x9d as to not interfere with the viewing of images. Chemical additives to impart hydrophobicity (sizing), wet strength, and dry strength may be used as needed. Inorganic filler materials such as TiO2, talc, and CaCO3 clays may be used to enhance optical properties and reduce cost as needed. Dyes, biocides, processing chemicals, etc. may also be used as needed. The paper may also be subject to smoothing operations such as dry or wet calendering, as well as to coating through an in-line or an off-line paper coater.
In another preferred lamination embodiment of this invention, the flange sheets used comprise high modulus polymers such as high density polyethylene, polypropylene, or polystyrene; their blends or their copolymers; that have been stretched and oriented. They may be filled with suitable filler materials as to increase the modulus of the polymer and enhance other properties such as opacity and smoothness. Some of the commonly used inorganic filler materials are talc, clays, calcium carbonate, magnesium carbonate, barium sulfate, mica, aluminum hydroxide (trihydrate), wollastonite, glass fibers and spheres, silica, various silicates, and carbon black. Some of the organic fillers used are wood flour, jute fibers, sisal fibers, polyester fibers, and the like. The preferred fillers are talc, mica, and calcium carbonate because they provide excellent modulus enhancing properties. Polymer flange sheets useful to this invention are of caliper between about 10 xcexcm and about 150 xcexcm, preferably between about 35 xcexcm and about 70 xcexcm.
Manufacturing Process
The elements of the invention can be made using several different manufacturing methods. The coextrusion, quenching, orienting, and heat setting of the element may be effected by any process which is known in the art for producing oriented sheet, such as by a flat sheet process or a bubble or tubular process. The flat sheet process involves extruding the blend through a slit die and rapidly quenching the extruded web upon a chilled casting drum so that the foam core component of the element and the polymeric integral flange components are quenched below their glass solidification temperature. The flange components may be extruded through a multiple stream die with the outer flange forming polymer streams not containing foaming agent, Alternatively, the surface of the foaming agent containing polymer may be cooled to prevent surface foaming and form a flange. The quenched sheet is then biaxially oriented by stretching in mutually perpendicular directions at a temperature above the glass transition temperature and below the melting temperature of the matrix polymers. The sheet may be stretched in one direction and then in a second direction or may be simultaneously stretched in both directions. After the sheet has been stretched, it is heat set by heating to a temperature sufficient to crystallize or anneal the polymers while restraining, to some degree, the sheet against retraction in both directions of stretching.
The element, while described as having preferably at least three layers of a foam core and a flange layer on each side, may also be provided with additional layers that may serve to change the properties of the element. Imaging elements could be formed with surface layers that would provide an improved adhesion or look.
These elements may be coated or treated after the coextrusion and orienting process or between casting and full orientation with any number of coatings which may be used to improve the properties of the sheets including printability, to provide a vapor barrier, to make them heat sealable, or to improve the adhesion to the support or to the photosensitive layers. Examples of this would be acrylic coatings for printability, coating polyvinylidene chloride for heat seal properties. Further examples include flame, plasma, or corona discharge treatment to improve printability or adhesion.
The element may also be made through the extrusion laminating process. Extrusion laminating is carried out by bringing together the paper or polymeric flange sheets of the invention and the foam core with application of an adhesive between them, followed by their being pressed in a nip such as between two rollers. The adhesive may be applied to either the flange sheets or the foam core prior to their being brought into the nip. In a preferred form, the adhesive is applied into the nip simultaneously with the flange sheets and the foam core. The adhesive may be any suitable material that does not have a harmful effect upon the element. A preferred material is polyethylene that is melted at the time it is placed into the nip between the foam core and the flange sheet. Addenda may also be added to the adhesive layer. Any know material used in the art to improve the optical performance of the system may be used. The use of TiO2 is preferred. During the lamination process also, it is desirable to maintain control of the tension of the flange sheets in order to minimize curl in the resulting laminated receiver support.
Foam Core Specifications
The suitable range in caliper of the foam core is from 25 xcexcm to 350 xcexcm. The preferred caliper range is between 50 xcexcm and 200 xcexcm because of the preferred overall caliper range of the element which lies between 100 xcexcm and 400 xcexcm. The range in density reduction of the foam core is from 20% to 95%. The preferred range in density reduction is between 40% and 70%. This is because it is difficult to manufacture a uniform product with very high density reduction (over 70%). Density reduction is the percent difference between solid polymer and a particular foam sample. It is also not economical to manufacture a product with density reduction less than 40%.
In another embodiment of this invention, the flange sheets used comprise paper on one side and a high modulus polymeric material on the other side. In another embodiment, an integral skin may be on one side and another skin laminated to the other side of the foam core.
The caliper of the paper and of the high modulus polymeric material is determined by the respective flexural modulus such that the overall stiffness of the imaging element lies within the preferred range, and the bending moment around the central axis is balanced to prevent excessive curl.
In addition to the stiffness and caliper, an imaging element needs to meet constraints in surface smoothness and optical properties such as opacity and colorimetry. Surface smoothness characteristics may be met during flange-sheet manufacturing operations such as during paper making or during the manufacture of oriented polymers like oriented polystyrene. Alternatively, it may be met by extrusion coating additional layer(s) of polymers such as polyethylene onto the flange sheets in contact with a textured chill-roll or similar technique known by those skilled in the art. Optical properties such as opacity and colorimetry may be met by the appropriate use of filler materials such as titanium dioxide and calcium carbonate and colorants, dyes and/or optical brighteners or other additives known to those skilled in the art. The fillers may be in the flange or an overcoat layer, such as polyethylene. Generally, base materials for color print imaging materials are white, possibly with a blue tint as a slight blue is preferred to form a preferred white look to whites in an image. Any suitable white pigment may be incorporated in the polyolefin layer such as, for example, titanium dioxide, zinc oxide, zinc sulfide, zirconium dioxide, white lead, lead sulfate, lead chloride, lead aluminate, lead phthalate, antimony trioxide, white bismuth, tin oxide, white manganese, white tungsten, and combinations thereof. The pigment is used in any form that is conveniently dispersed within the flange or resin coat layers. The preferred pigment is titanium dioxide. In addition, suitable optical brightener may be employed in the polyolefin layer including those described in Research Disclosure, Vol. No. 308, December 1989, Publication 308119, Paragraph V, page 998.
In addition, it may be necessary to use various additives such as antioxidants, slip agents, or lubricants, and light stabilizers in the plastic elements as well as biocides in the paper elements. These additives are added to improve, among other things, the dispersibility of fillers and/or colorants, as well as the thermal and color stability during processing and the manufacturability and the longevity of the finished article. For example, the polyolefin coating may contain antioxidants such as 4,4xe2x80x2-butylidene-bis(6-tert-butyl-meta-cresol), di-lauryl-3,3xe2x80x2-thiopropionate, N-butylated-p-aminophenol, 2,6-di-tert-butyl-p-cresol, 2,2-di-tert-butyl-4-methyl-phenol, N,N-disalicylidene-1,2-diaminopropane, tetra(2,4-tert-butylphenyl)-4,4xe2x80x2-diphenyl diphosphonite, octadecyl 3-(3xe2x80x2,5xe2x80x2-di-tert-butyl-4xe2x80x2-hydroxyphenyl propionate), combinations of the above, and the like; heat stabilizers, such as higher aliphatic acid metal salts such as magnesium stearate, calcium stearate, zinc stearate, aluminum stearate, calcium palmitate, zirconium octylate, sodium laurate, and salts of benzoic acid such as sodium benzoate, calcium benzoate, magnesium benzoate and zinc benzoate; light stabilizers such as hindered amine light stabilizers (HALS), of which a preferred example is poly {[6-[(1,1,3,3-tetramethylbutylamino}-1,3,5-triazine-4-piperidinyl)-imino]-1,6-hexanediyl[{2,2,6,6-tetramethyl-4-piperdinyl)imino]}(Chimassorb 944 LD/FL).
Used herein, the phrase xe2x80x98imaging elementxe2x80x99 comprises an imaging support as described above along with an image receiving layer as applicable to multiple techniques governing the transfer of an image onto the imaging element. Such techniques include thermal dye transfer, electrophotographic printing, or ink jet printing, as well as a support for photographic silver halide images. As used herein, the phrase xe2x80x9cphotographic elementxe2x80x9d is a material that utilizes photosensitive silver halide in the formation of images.
The thermal dye image-receiving layer of the receiving elements of the invention may comprise, for example, a polycarbonate, a polyurethane, a polyester, polyvinyl chloride, poly(styrene-co-acrylonitrile), poly(caprolactone), or mixtures thereof. The dye image-receiving layer may be present in any amount that is effective for the intended purpose. In general, good results have been obtained at a concentration of from about 1 to about 10 g/m2. An overcoat layer may be further coated over the dye-receiving layer, such as described in U.S. Pat. No. 4,775,657 of Harrison et al.
Dye-donor elements that are used with the dye-receiving element of the invention conventionally comprise a support having thereon a dye containing layer. Any dye can be used in the dye-donor employed in the invention, provided it is transferable to the dye-receiving layer by the action of heat. Especially good results have been obtained with sublimable dyes. Dye donors applicable for use in the present invention are described, e.g., in U.S. Pat. Nos. 4,916,112; 4,927,803; and 5,023,228. As noted above, dye-donor elements are used to form a dye transfer image. Such a process comprises image-wise-heating a dye-donor element and transferring a dye image to a dye-receiving element as described above to form the dye transfer image. In a preferred embodiment of the thermal dye transfer method of printing, a dye donor element is employed which compromises a poly(ethylene terephthalate) support coated with sequential repeating areas of cyan, magenta, and yellow dye, and the dye transfer steps are sequentially performed for each color to obtain a three-color dye transfer image. When the process is only performed for a single color, then a monochrome dye transfer image is obtained.
Thermal printing heads which can be used to transfer dye from dye-donor elements to receiving elements of the invention are available commercially. There can be employed, for example, a Fujitsu Thermal Head (FTP-040 MCS001), a TDK Thermal Head F415 HH7-1089, or a Rohm Thermal Head KE 2008-F3. Alternatively, other known sources of energy for thermal dye transfer may be used, such as lasers as described in, for example, GB No. 2,083,726A.
A thermal dye transfer assemblage of the invention comprises (a) a dye-donor element, and (b) a dye-receiving element as described above, the dye-receiving element being in a superposed relationship with the dye-donor element so that the dye layer of the donor element is in contact with the dye image-receiving layer of the receiving element.
When a three-color image is to be obtained, the above assemblage is formed on three occasions during the time when heat is applied by the thermal printing head. After the first dye is transferred, the elements are peeled apart. A second dye-donor element (or another area of the donor element with a different dye area) is then brought in register with the dye-receiving element and the process repeated. The third color is obtained in the same manner.
The electrographic and electrophotographic processes and their individual steps have been well described in the prior art. The processes incorporate the basic steps of creating an electrostatic image, developing that image with charged, colored particles (toner), optionally transferring the resulting developed image to a secondary substrate, and fixing the image to the substrate. There are numerous variations in these processes and basic steps; the use of liquid toners in place of dry toners is simply one of those variations.
The first basic step, creation of an electrostatic image, can be accomplished by a variety of methods. The electrophotographic process of copiers uses imagewise photodischarge, through analog or digital exposure, of a uniformly charged photoconductor. The photoconductor may be a single-use system, or it may be rechargeable and reimageable, like those based on selenium or organic photoreceptors.
In one form, the electrophotographic process of copiers uses imagewise photodischarge, through analog or digital exposure, of a uniformly charged photoconductor. The photoconductor may be a single-use system, or it may be rechargeable and reimageable, like those based on selenium or organic photoreceptors.
In an alternate electrographic process, electrostatic images are created ionographically. The latent image is created on dielectric (charge-holding) medium, either paper or film. Voltage is applied to selected metal styli or writing nibs from an array of styli spaced across the width of the medium, causing a dielectric breakdown of the air between the selected styli and the medium. Ions are created, which form the latent image on the medium.
Electrostatic images, however generated, are developed with oppositely charged toner particles. For development with liquid toners, the liquid developer is brought into direct contact with the electrostatic image. Usually a flowing liquid is employed to ensure that sufficient toner particles are available for development. The field created by the electrostatic image causes the charged particles, suspended in a nonconductive liquid, to move by electrophoresis. The charge of the latent electrostatic image is thus neutralized by the oppositely charged particles. The theory and physics of electrophoretic development with liquid toners are well described in many books and publications.
If a reimageable photoreceptor or an electrographic master is used, the toned image is transferred to paper (or other substrate). The paper is charged electrostatically, with the polarity chosen to cause the toner particles to transfer to the paper. Finally, the toned image is fixed to the paper. For self-fixing toners, residual liquid is removed from the paper by air-drying or heating. Upon evaporation of the solvent, these toners form a film bonded to the paper. For heat-fusible toners, thermoplastic polymers are used as part of the particle. Heating both removes residual liquid and fixes the toner to paper.
When used as ink jet imaging media, the recording elements or media typically comprise a substrate or a support material having on at least one surface thereof an ink-receiving or image-forming layer. If desired, in order to improve the adhesion of the ink receiving layer to the support, the surface of the support may be corona-discharge-treated prior to applying the solvent-absorbing layer to the support or, alternatively, an undercoating, such as a layer formed from a halogenated phenol or a partially hydrolyzed vinyl chloride-vinyl acetate copolymer, can be applied to the surface of the support. The ink receiving layer is preferably coated onto the support layer from water or water-alcohol solutions at a dry thickness ranging from 3 to 75 micrometers, preferably 8 to 50 micrometers.
Any known ink jet receiver layer can be used in combination with the external polyester-based barrier layer of the present invention. For example, the ink receiving layer may consist primarily of inorganic oxide particles such as silicas, modified silicas, clays, aluminas, fusible beads such as beads comprised of thermoplastic or thermosetting polymers, non-fusible organic beads, or hydrophilic polymers such as naturally-occurring hydrophilic colloids and gums such as gelatin, albumin, guar, xantham, acacia, chitosan, starches and their derivatives, and the like; derivatives of natural polymers such as functionalized proteins, functionalized gums and starches, and cellulose ethers and their derivatives; and synthetic polymers such as polyvinyloxazoline, polyvinylmethyloxazoline, polyoxides, polyethers, poly(ethylene imine), poly(acrylic acid), poly(methacrylic acid), n-vinyl amides including polyacrylamide and polyvinylpyrrolidone, and poly(vinyl alcohol), its derivatives and copolymers; and combinations of these materials. Hydrophilic polymers, inorganic oxide particles, and organic beads may be present in one or more layers on the substrate and in various combinations within a layer.
A porous structure may be introduced into ink receiving layers comprised of hydrophilic polymers by the addition of ceramic or hard polymeric particulates, by foaming or blowing during coating, or by inducing phase separation in the layer through introduction of non-solvent. In general, it is preferred for the base layer to be hydrophilic, but not porous. This is especially true for photographic quality prints, in which porosity may cause a loss in gloss. In particular, the ink receiving layer may consist of any hydrophilic polymer or combination of polymers with or without additives as is well known in the art.
If desired, the ink receiving layer can be overcoated with an ink-permeable, anti-tack protective layer such as, for example, a layer comprising a cellulose derivative or a cationically-modified cellulose derivative or mixtures thereof. An especially preferred overcoat is poly xcex2-1,4-anhydro-glucose-g-oxyethylene -g-(2xe2x80x2-hydroxypropyl)-N,N-dimethyl-N-dodecylammonium chloride. The overcoat layer is non porous, but is ink permeable and serves to improve the optical density of the images printed on the element with water-based inks. The overcoat layer can also protect the ink receiving layer from abrasion, smudging, and water damage. In general, this overcoat layer may be present at a dry thickness of about 0.1 to about 5 xcexcm, preferably about 0.25 to about 3 xcexcm.
In practice, various additives may be employed in the ink receiving layer and overcoat. These additives include surface active agents such as surfactant(s) to improve coatability and to adjust the surface tension of the dried coating, acid or base to control the pH, antistatic agents, suspending agents, antioxidants, hardening agents to cross-link the coating, antioxidants, UV stabilizers, light stabilizers, and the like. In addition, a mordant may be added in small quantities (2%-10% by weight of the base layer) to improve waterfastness. Useful mordants are disclosed in U.S. Pat. No. 5,474,843.
The layers described above, including the ink receiving layer and the overcoat layer, may be coated by conventional coating means onto a transparent or opaque support material commonly used in this art. Coating methods may include, but are not limited to, blade coating, wound wire rod coating, slot coating, slide hopper coating, gravure, curtain coating, and the like. Some of these methods allow for simultaneous coatings of both layers, which is preferred from a manufacturing economic perspective.
The DRL (dye receiving layer) is coated over the tie layer or TL at a thickness ranging from 0.1-10 xcexcm, preferably 0.5-5 xcexcm. There are many known formulations which may be useful as dye receiving layers. The primary requirement is that the DRL is compatible with the inks which it will be imaged so as to yield the desirable color gamut and density. As the ink drops pass through the DRL, the dyes are retained or mordanted in the DRL, while the ink solvents pass freely through the DRL and are rapidly absorbed by the TL. Additionally, the DRL formulation is preferably coated from water, exhibits adequate adhesion to the TL, and allows for easy control of the surface gloss.
For example, Misuda et al in U.S. Pat. Nos. 4,879,166; 5,264,275; 5,104,730; 4,879,166, and Japanese Pat. Nos. 1,095,091; 2,276,671; 2,276,670; 4,267,180; 5,024,335; and 5,016,517 disclose aqueous based DRL formulations comprising mixtures of psuedo-bohemite and certain water soluble resins. Light in U.S. Pat. Nos. 4,903,040; 4,930,041; 5,084,338; 5,126,194; 5,126,195; and 5,147,717 discloses aqueous-based DRL formulations comprising mixtures of vinyl pyrrolidone polymers and certain water-dispersible and/or water-soluble polyesters, along with other polymers and addenda. Butters et al in U.S. Pat. Nos. 4,857,386 and 5,102,717 disclose ink-absorbent resin layers comprising mixtures of vinyl pyrrolidone polymers and acrylic or methacrylic polymers. Sato et al in U.S. Pat. No. 5,194,317 and Higuma et al in U.S. Pat. No. 5,059,983 disclose aqueous-coatable DRL formulations based on poly(vinyl alcohol). Iqbal in U.S. Pat. No. 5,208,092 discloses water-based IRL formulations comprising vinyl copolymers which are subsequently cross-linked. In addition to these examples, there may be other known or contemplated DRL formulations which are consistent with the aforementioned primary and secondary requirements of the DRL, all of which fall under the spirit and scope of the current invention.
The preferred DRL is 0.1-10 micrometers thick and is coated as an aqueous dispersion of 5 parts alumoxane and 5 parts poly(vinyl pyrrolidone). The DRL may also contain varying levels and sizes of matting agents for the purpose of controlling gloss, friction, and/or fingerprint resistance, surfactants to enhance surface uniformity and to adjust the surface tension of the dried coating, mordanting agents, antioxidants, UV absorbing compounds, light stabilizers, and the like.
Although the ink-receiving elements as described above can be successfully used to achieve the objectives of the present invention, it may be desirable to overcoat the DRL for the purpose of enhancing the durability of the imaged element. Such overcoats may be applied to the DRL either before or after the element is imaged. For example, the DRL can be overcoated with an ink-permeable layer through which inks freely pass. Layers of this type are described in U.S. Pat. Nos. 4,686,118; 5,027,131; and 5,102,717. Alternatively, an overcoat may be added after the element is imaged. Any of the known laminating films and equipment may be used for this purpose. The inks used in the aforementioned imaging process are well known, and the ink formulations are often closely tied to the specific processes, i.e., continuous, piezoelectric, or thermal. Therefore, depending on the specific ink process, the inks may contain widely differing amounts and combinations of solvents, colorants, preservatives, surfactants, humectants, and the like. Inks preferred for use in combination with the image recording elements of the present invention are water-based, such as those currently sold for use in the Hewlett-Packard Desk Writer 560C printer. However, it is intended that alternative embodiments of the image-recording elements as described above, which may be formulated for use with inks which are specific to a given ink-recording process or to a given commercial vendor, fall within the scope of the present invention.
Smooth opaque paper bases are useful in combination with silver halide images because the contrast range of the silver halide image is improved, and show through of ambient light during image viewing is reduced. The preferred photographic element of this invention is directed to a silver halide photographic element capable of excellent performance when exposed by either an electronic printing method or a conventional optical printing method. An electronic printing method comprises subjecting a radiation sensitive silver halide emulsion layer of a recording element to actinic radiation of at least 10xe2x88x924 ergs/cm2 for up to 100 xcexc seconds duration in a pixel-by-pixel mode wherein the silver halide emulsion layer is comprised of silver halide grains as described above. A conventional optical printing method comprises subjecting a radiation sensitive silver halide emulsion layer of a recording element to actinic radiation of at least 10xe2x88x924 ergs/cm2 for 10xe2x88x923 to 300 seconds in an imagewise mode wherein the silver halide emulsion layer is comprised of silver halide grains as described above. This invention in a preferred embodiment utilizes a radiation-sensitive emulsion comprised of silver halide grains (a) containing greater than 50 mole percent chloride based on silver, (b) having greater than 50 percent of their surface area provided by {100} crystal faces, and (c) having a central portion accounting for from 95 to 99 percent of total silver and containing two dopants selected to satisfy each of the following class requirements: (i) a hexacoordination metal complex which satisfies the formula:
[ML6]nxe2x80x83xe2x80x83(I)
wherein n is zero, xe2x88x921, xe2x88x922, xe2x88x923, or xe2x88x924; M is a filled frontier orbital polyvalent metal ion, other than iridium; and L6 represents bridging ligands which can be independently selected, provided that at least four of the ligands are anionic ligands, and at least one of the ligands is a cyano ligand or a ligand more electronegative than a cyano ligand; and (ii) an iridium coordination complex containing a thiazole or substituted thiazole ligand. Preferred photographic imaging layer structures are described in EP Publication 1 048 977. The photosensitive imaging layers described therein provide particularly desirable images on the base of this invention.
This invention is directed towards a photographic recording element comprising a support and at least one light sensitive silver halide emulsion layer comprising silver halide grains as described above.
The following examples illustrate the practice of this invention. They are not intended to be exhaustive of all possible variations of the invention. Parts and percentages are by weight unless otherwise indicated.